The new farms hit by the highly contagious disease are in Haiphong and Thanh Hoa provinces, 100 km to the east and 160 km to the south of Hanoi, respectively, according to a report in the Nong Nghiep Vietnam newspaper, run by the Ministry of Agriculture.

Reuters reports that the virus is spreading at locations around the capital city of Hanoi, which will host the second summit between US President Donald Trump and North Korean leader Kim Jong Un later this week.

Authorities have culled all the pigs at the farms, including 226 pigs at the farm in Thanh Hoa province.

Authorities said last week that African swine fever had been detected at three farms in the Thai Binh and Hung Yen provinces, the first confirmed cases of the disease in the Southeast Asian country.

The disease, which is incurable in pigs but harmless to humans, has spread rapidly across neighbouring China since August.

Pork accounts for three quarters of total meat consumption in Vietnam, a country of 95 million people where most of its 30 million farm-raised pigs are consumed domestically.

"Our first priority at the moment is to contain the disease from spreading," Deputy Minister of Agriculture Phung Duc Tien was quoted as saying in the report.

"If it spreads further, it will be very difficult to contain," Tien said.